Pain, fear and stress are likely to be experienced by fish in similar ways as in tetrapods (amphibians, reptiles, birds and mammals)Credit: National Oceanic and Atmospheric Administration/Department of Commerce.

Sentience is about the inner life of an animal, and a sentient animal has capacity to suffer fear, pain or distress as well as a sense of well-being. Evidence that fish are sentient has been sufficient to achieve international recognition that their welfare matters. The policy statement of the World Organisation for Animal Health (OIE) states:

“The use of fish carries with it an ethical responsibility to ensure the welfare of such animals to the greatest extent practicable.”

“The balance of the evidence indicates that some fish species have the capacity to experience pain”

and that

“Responses of fish, of some species and under certain situations, suggest that they are able to experience fear”.

The Medway Report

The Medway Report, commissioned by the RSPCA and published in 1980, concluded that:

“In the light of evidence reviewed … it is recommended that, where considerations of welfare are involved, all vertebrate animals (i.e., mammals birds, reptiles, amphibians and fish) should be regarded as equally capable of suffering to some degree or another, without distinction between ‘warm-blooded’ and ‘cold blooded’ members.”

Researching fish sentience

Social intelligence in fishFish are "steeped in social intelligence, pursuing Machiavellian strategies of manipulation, punishment and reconciliation, exhibiting stable cultural traditions, and co-operating to inspect predators and catch food"Credit: National Oceanic and Atmospheric Administration/Department of Commerce

In the last 20 years, animal welfare science has developed into a scientific field in its own right, and the evidence for fish sentience has grown. Because animal consciousness cannot be measured directly, animal welfare scientists look for anatomical, physiological and behavioural evidence as indicators of sentience or suffering. A collection of articles on fish learning was published in a special edition of “Fish and fisheries” in 2003. The BBC news website reported this (see BBC reports fish intelligence) saying that, according to scientists, fish

“do not deserve their reputation as the dim-wits of the animal kingdom”.

An example of fish learning can be seen in the following video clip. “Comet” the goldfish has been trained by his owners to perform a number of tricks for food rewards e.g. he will “fetch just like dogs do”.

Of key importance in animal welfare is that capacity to experience pain, fear and distress. Professor Donald Broom, of the University of Cambridge, sums up the case for fish feeling pain:

“There are some differences in sensory functioning between fish and mammals because fish live in water but the pain system of fish is very similar to that of birds and mammals. Fish have pain receptor cells, nociceptive neuronal pathways, specialized transmitter substances, electrophysiological responses to cuts, bruises and electric shocks, behavioural avoidance, learned avoidance of places where they had unpleasant experiences and processing systems in the brain which parallel those in birds and mammals. Hence at least some aspects of pain as we know it must be felt by fish.”

Fish feel pain"at least some aspects of pain as we know it must be felt by fish."Credit: Courtesy of United Nations Food and Agriculture Organization.

Herring caught in the crushNational Oceanic and Atmospheric Administration/Department of Commerce.

Fish have brain structures capable of feeling fear and pain

Like that of other vertebrates, the fish brain consists of the forebrain (i.e. telencephalon and diencephalon), midbrain and hindbrain. The fish brain is not identical to the mammalian brain. It is smaller and fish do not have the extensive cerebral cortex seen in the forebrain of mammals. This is a laminated structure which covers the telencephalon. It has been argued that because fish do not possess this laminated structure (a “neocortex”), they must therefore be incapable of experiencing pain. However, there is good reason to believe that fish do experience pain and fear without this particular structure.

Convergent evolution. The brains of sentient animals can perform similar functions, without necessarily following the same design. An example of convergent evolution is seen in the dolphin brain, which is organised in a "fundamentally different pattern" to those of primates. Yet these animals have great cognitive abilities, seen elsewhere only in humans and great apes.Credit: US National Oceanic and Atmospheric Administration.

It is known that the same brain function can be served by different brain structures in different groups of animals, e.g. cognitive functions in birds and mammals (visual stimuli are processed by part of the cerebral cortex in mammals but by the midbrain optic tectum in birds). Another example is seen in dolphins, highly intelligent animals whose brain is organized in a fundamentally different way to that of primates. It is also a matter of some scientific debate whether human consciousness is a function of the neotcortex alone, or restricted to any single area of the brain.

There is evidence that the fish forebrain contains within it several brain structures that perform similar functions to those associated with pain and fear in higher vertebrates. For example, the dorsomedial (Dm) and dorsolateral (Dl) telencephalon are thought to perform the same functions as the amygdala and hippocampus respectively in mammals. The amygdala is important in arousal and emotions, particularly fear responses, while the hippocampus is involved in memory and learning of spatial relationships. Damage to the Dm area in fish has been observed to impair the fear response without affecting spacial learning, and vice versa for damage to the Dl area.

Critics of fish sentience focus on the structural differences between the brain of fish and that of mammals. Through convergent evolution, different species can develop the same function through anatomical structures that may be quite different. For example, there is good evidence that some invertebrates, such as crustaceans, have the capacity for fear and pain, despite the lack a vertebrate pain system at all. Research conducted by Professor Robert Elwood and Mirjam Appel at Queens University, Belfast, found that hermit crabs reacted adversely to electric shocks but also seemed to try to avoid future shocks, suggesting that they recalled the past ones. This research was widely reported (see crabs ‘sense and remember pain’). Animal welfare scientists such as Professor Robert Elwood and Professor Donald Broom have argued that the welfare of these animals should also receive some legal protection. The invertebrates with the most complex brains are the cephalopods (including octopus and squid), which can solve maze puzzles and remember the solutions. Cepahalopods appear to show strong emotions that are signaled by profound changes in colour. In 1993, the UK legislation governing the use of animals in scientific research was amended to include the common octopus.

The AHAW panel, commissioned by the EU Commission and referred to above, concluded:

“There is scientific evidence to support the assumption that some fish species have brain structures potentially capable of experiencing pain and fear”.

As Professor John Webster, of the University of Bristol, argues that since all or nearly all the evidence points in the direction of fish feeling pain:

“The claim that fish ‘do not have the right sort of brain’ to feel pain can no longer be called scientific”

and that

“to say that a fish cannot feel pain because it doesn’t have a neocortex is like saying it cannot breathe because it doesn’t have lungs”.

For more detail on the evidence for pain and fear in fish, visit the subpages below.

Fish feel stress

Animal suffering is wider than pain and fear. The AHAW panel report that the stress physiology in fish is “directly comparable to that of higher vertebrates” and manifested as primary, secondary and tertiary stress responses. The primary response includes the release of hormones e.g. cortisol.

In a number of studies, measurement of physiological variables (such as cortisol) and adverse behaviour have shown that fish suffer stress when caught (for example in gill nets and purse seine nets, in fish traps and by hooks) and when subjected to live chilling and asphyxiation.

In her book “Do fish feel pain?”, Dr. Victoria Braithwaite brings the science behind the debate around pain in fish into the open. She describes the many different pieces of evidence that together build up a picture of fish as animals that, she concludes, “have the mental capacity to feel pain”. She argues, on the basis of the evidence, that:

“I see no logical reason why we should not extend to fish the same welfare considerations that we currently extend to birds and mammals”.

Conclusion

Most of what is known about human pain is from self-reporting. Because a fish cannot report to us what it is feeling, it may be that scientific method cannot prove, in an absolute sense, that fish feel pain. Just as it cannot be totally proven that babies, or even you and I, can feel pain. The balance of evidence, together with what is understood about evolution and the biological purpose of fear and pain, indicates that fish do feel fear, pain and distress and, for humane reasons, the benefit of any doubt should be given to avoiding suffering.